Several fiber optic thermometers based on the fluorescence lifetime of the Cr3+ doped crystal materials are reviewed. The work examines the temperature dependences of the Cr3 fluorescence lifetimes in various host materials according to the experimental data obtained by the authors and other groups of workers classifies such dependences by the strengths of the crystal fields of the host materials and gives corresponding mathematical models to describe such temperature dependences. The construction of the Cr3 fluorescence lifetime based thermometers is discussed from the view point of optical arrangement the temperature probes and the signal processing techniques for the detection of fluorescence lifetimes used in various thermometer schemes. Several examples are presented and finally the question of calibration of the thermometers using the given mathematical models and other concerns are addressed.